The present invention relates to calibrating transmitters and receivers of an ultrasound imaging system. In particular, amplitude and phase adjustments for one channel relative to other channels are provided by calibration.
By measuring amplitude and/or phase differences between different channels of an ultrasound imaging system, system induced differences or artifacts may be minimized. For example, transmit wave forms of one channel are increased in amplitude, decreased in amplitude, or phase rotated to account for system induced differences relative to another channel. Likewise, receive performance may also be calibrated. Calibration may allow for images with less system induced noise. Calibration ascertains output amplitude and phase shifts between transmitters, or ascertains voltage gain and phase shift between receivers.
Hardware is positioned within the imaging system adjacent to a transducer port for calibration. The coupling node for calibration is used to inject signals of well-defined properties for receiver calibration and precisely monitor signals for transmit calibration. FIG. 1 shows one embodiment of a calibration system 10 for ultrasound imaging. A plurality of transmitters 12 and receivers 14 connect with respective transmit and receive lines 16. For use in imaging, pairs of diodes 20 connect to respective transmit and receive lines 16. One diode 20 of each pair connects with a positive high voltage source, and the other diode 20 of each pair connects with a negative high voltage source. The positive and negative voltage sources act to limit a possible voltage output on each transmit and receive line to avoid injury.
A calibration node 18 also connects with each of the transmit and receive lines 16. The calibration node 18 is used for calibrating prior to imaging. The calibration node 18 includes resistive or capacitive components. For example, each transmit and receive line 16 is connected together through low valued capacitors (e.g., less then 50 picofarads) to a common node or conductor. For example, a printed wiring board trace is run under the transmit and receive lines to form the capacitive common node. The common node is connected to ground through a larger capacitor (e.g., greater than 1,000 picofarads) in order to limit the amount of unwanted crosstalk introduced from line-to-line capacitance. For calibrating receivers, a signal is generated on the common node, and each transmit and receive lines 16 is measured sequentially using the receivers 14. Amplitude and phase differences are identified from the measured signal. For transmit calibration, the transmitters 12 sequentially generate transmit waveforms on the transmit and receive line 16, and an amplitude and phase is measured at the common node. However, this capacitive calibration node may not be suitable for systems operating at high frequencies, such as greater than 20 MHz. As the frequency increases, the impedance of a capacitor decreases. The capacitive impedance to ground decreases within the calibration node 18, which may cause discontinuity in the transmission lines, likely degrading system performance.
In another approach, the calibration node 18 is a resistive device. Each of the transmit and receive lines 16 are connected together through resistors, such as on the order of less than 1 kilo-ohms. The common node is then connected to ground through a relatively small valued resistor, such as less than 100 ohms, in order to limit the amount of unwanted crosstalk introduced from line-to-line resistance. The common node is used as discussed above to determine relative amplitude and phases for both transmit and receive operation. In normal imaging operation, the resistive based calibration node 18 may operate over a wide range of frequency signals, including greater than 20 megahertz. However, in calibration mode, as the frequency increases, the resistors increasingly act as low pass filters because of stray capacitance to ground. Degraded calibration performance may result. Substantial transmit power loss over all frequencies may also result.